A crusty problem for Darwin
Darwin’s major water authority, Power and Water Corporation, treats the vast majority of the Northern Territory capital’s sewage and wastewater. The system, installed many years ago but continually upgraded, was experiencing crusting problems in its major sewage wet wells.
Not all the wells had mixers, and those that did were in need of replacement. The issue meant more cyclic maintenance and downtime because crusting – which can be 15 to 20 centimetres thick – was causing three issues:
The crust blanket was fouling the floats that provide each wet-well’s level control devices, and necessitated the use of high pressure hoses – a messy, costly and time consuming procedure – to break up the crust. This resulted in the wet-wells having to be pumped down under manual control, causing some foreign matter/solids blockages in the lower levels of the well.
It also caused an odour within the station.
From a public point of view, there was also potential for this crusting to cause a nasty odour more generally.
Says Power and Water Corporation’s Manager Water Operations, Maintenance and Monitoring, Norm Cramp: “The combined issues were causing increased cyclic maintenance which in turn added to our operational costs.
We decided to examine ways in which we could prevent the crust from forming, as this was the source of the problems.
“A number of the larger wells had mixer pumps, but these had reached their use-by date. Some had been in service for more than seven years, and operating in the highly corrosive and aggressive conditions that one expects in sewage wet wells. We felt there must be a way in which we could better ‘engineer out’ the problem.”
The Grundfos Solution
Mr. Cramp says after considerable research and cost analysis, Power and Water Corporation decided to use Grundfos AMD mixers.
“We chose Grundfos mixers because of their design, their stainless steel construction and their reputation as being robust and able to handle tough workloads. We had also dealt with Grundfos in the past, and appreciated the company’s after sales service and reliability.”
The systems were installed about three years ago in conjunction with local Darwin Grundfos dealer, AG Drilling Pty Ltd, of Darwin, which also provided the pumps.
Owner of AGD, Kevin Sneyd, says that while the installations were easy, they were time consuming and fiddly.
“The wet wells don’t provide much room for working,” he says. “So we had to be issued with a confined spaces work permit - standard practice when working in such environments. Wet wells are hardly easy workplaces and vary in size from about six to eight metres in diameter with depths of between about nine and twelve metres.
“Each installation took about three to four days. The smaller wells have single AMD mixers, while the larger ones have dual AMD mixers. During the installation periods there was no down time or any significant operational problems, as the station subject to the work was by-passed by installing a pump in an adjacent collection pit and pumping the waste from there.”
Mr. Cramp says the system, since its installation three years ago, has been working very well.
“We have been very pleased with the result,” he says. “Our cyclic maintenance costs have reduced and the Grundfos AMD mixers have performed very satisfactorily.
“They have been functioning effectively and certainly do an excellent job in breaking up the sewage crusting.
“Each of the stations pumps about 180 litres per second over an eight hour day, so the mixers are standing up to very rigorous use - as we had hoped they would.”